DESCRIPTION: (Applicant's Abstract) This study addresses the question: what are necessary and sufficient conditions for effective treatment services for homeless substance abusers? It refines an ongoing innovative day treatment program for dually diagnosed, cocaine-abusing homeless persons, including work and housing interventions. It enhances aftercare and studies the critical variable of program-provided housing. It establishes a cohort of 225 cocaine-abusing homeless persons, including work and housing interventions. It enhances aftercare, and studies the critical variable of program-provided housing. It establishes a cohort of 225 cocaine-abusing homeless persons, with DSM III-R Axis I Mental Disorders in addition to Substance Use Disorders, then randomly assigns them to 3 groups of 75 based on the main independent variable, housing intervention. The studied variable compares abstinent-contingent housing, non-abstinent contingent housing and no housing, all in the context of intensive day treatment, vocational interventions, and aftercare derived from successful intervention in two previous studies by the research team. Except for the studied variable, all groups will share in the same interventions by the same counselors in 3 phases: Phase I, mo. 1-2 intensive daily day treatment; Phase II, mo. 3-6, aftercare 3 days/wk; Phase III, mo. 7-12, aftercare 2 days/wk. Subjects will be assessed with the previously used battery at 2, 6, 12 and 18 months on primary outcomes of alcohol and substance use, homelessness and employment. A cost-effectiveness/benefits analysis for primary outcomes will be completed. Secondary outcomes of psychiatric functioning and HIV risk will be measured. The study investigates whether day treatment plus program-provided abstinent-contingent housing is necessary and sufficient for maximum treatment effects or, if day treatment plus non-abstinent contingent-provided housing or day treatment alone are sufficient for treating substance abusing homeless. Results will provide important knowledge on how to most cost-effectively provide treatment services for this difficult population, knowledge which has important clinical, economic, and public policy implications.